A so-called progressive-power lens having no dividing line has been widely used as a spectacle lens for correcting presbyopia. A recent proposal has been a so-called inside surface progressive lens, in which a progressive surface or a curved surface is formed by synthesizing a progressive surface and a toric surface provided on an eyeball-side concave surface. The inside surface progressive lens is capable of reducing oscillation and distortion, which are drawbacks of the progressive-power lens, offering considerable improvements in optical characteristics. A related-art reference concerning a technique for creating a non-axisymmetrical and aspherical surface, such as a progressive-surface on a concave surface of a spectacle lens, has been disclosed in Japanese Patent No. 3,367,102 and JP-A-10-175149.
An aspherical surface processing apparatus, for creating a non-axisymmetrical and aspherical surface by controlling three axes, successively positions a rotational tool on a predetermined location by using three axes of an X-axis table, a Y-axis table, and a workpiece rotating means. Thus, it creates a lens shape in accordance with a lens shape design by grinding and cutting.
The following is an outline of the control method: calculating a rotation position of a workpiece by an encoder while rotating the workpiece, and controlling the three axes of the X-axis table, the Y-axis table, and the workpiece rotating means in synchronism with the rotation position.
A normal control processing method, as a conventional shape-creation control method using the aspherical surface processing apparatus of this type, is herein described with reference to FIGS. 12A, 12B, 13, and 14. FIG. 12A is a schematic view illustrating a processed surface of a lens processed by the normal control processing method. FIG. 12A is a front view of the lens, while FIG. 12B is a cross-sectional view of the lens taken along a line B–B′ in FIG. 12A. FIG. 13 is a conceptual view showing the normal control processing method. FIG. 14 is a conceptual view showing a center position of a rotational tool in an X-axis direction in the normal control processing method.
Numerical value data used for numerical control in the normal control processing method is now explained using an arbitrary point Qx shown in FIG. 12A. In the numerical value data for numerical control in the normal control processing method, a spiral specified by a feed pitch P from a periphery of a round lens to a rotation center is supposed, and coordinate values of respective intersections of lines radially extending from the rotation center of the lens at intervals of a predetermined angle and the spiral are given by a rotation angle (Θ) and a distance from the rotation center (radius Rx) of the lens. Also, a height (y) in accordance with a surface shape in the Y-axis direction passing through not-shown respective intersections is obtained. These three values are determined as coordinate values (Θ, Rx, y) of a processing point.
A toric surface is a curved surface which includes a curve having the minimum curvature along a line A–A′ (base curve), and a curve having the maximum curvature along a line B–B′ (cross curve) perpendicular to the line A–A′. When the difference in curvature between the base curve and the cross curve is large, a cross section taken along the cross curve becomes a curve surface configuration having both extremely thick end portions and a thin central portion as illustrated in FIG. 12B. A rotational tool 214 reciprocates between a height of the portion having the minimum thickness and a height of the portion having the maximum thickness, i.e., in the Y-axis direction every time the rotational tool 214 rotates 180 degrees. For example, the lens rotates 90 degrees from the A–A′ cross section to the B–B′ cross section as illustrated in FIG. 13, the rotational tool 214 shifts in the plus direction of the Y-axis from an arbitrary processing point Qn of the portion having the minimum thickness to an arbitrary processing point Qnm at the maximum height.
The tip of the rotational tool 214 used for the grinding and cutting has a circular arc cross section (hereinafter referred to as “round shape”). In the normal control, for example, the center of the round shape portion of the tip of the rotational tool 214 is positioned in a normal direction established at the processing point Qn of the lens.
More specifically, at the arbitrary processing point Qn on the curve having the minimum thickness (base curve, A–A′ cross section), a center point Pn of the rotational tool 214 is positioned in the normal direction established at the processing point Qn. At an arbitrary point Qnm on the curve having the maximum height (cross curve, B–B′ cross section) with the lens rotated 90 degrees from the processing point Qn, a center point Pnm of the rotational tool 214 is positioned in the normal direction established at the processing point Qnm. At this stage, the processing point Qnm shifts a quarter of a pitch from the processing point Qn toward the center in the X-axis direction. The rotational tool 214 moves ΔY in the plus direction of the Y-axis and Xm toward the center in the X-axis direction relatively during the shift from the processing point Qn to the processing point Qnm. At an arbitrary processing point Qnr on the curve having the minimum height (base curve, A–A′ cross section) with the lens rotated further 90 degrees, the rotational tool 214 shifts in the minus direction of the Y-axis though not shown in the figure. At this stage, since the speed of the outward movement with reduction in thickness is larger than the speed of the movement to the center of the feed pitch, the rotational tool 214 relatively shifts Xr toward the periphery as illustrated in FIG. 14. More specifically, the cross curve on the B–B′ cross section provides an inflection point at which the shift direction changes to the opposite. Thus, the movement direction of the rotational tool 214 alters to the opposite on the boundary cross curve on the B–B′ cross section, so that the rotational tool 214 reciprocates in the Y-axis and X-axis directions.
In the normal control processing method, the processing points are located at the intersections of the spiral and the radially extending lines as illustrated in FIG. 12A, and the center position of the tip of the rotational tool 214 is positioned in the normal directions established at the processing points. More specifically, in the normal control processing method, the rotational tool 214 grinds and cuts a workpiece while forming a complicated zigzag spiral track by repeatedly altering the movement direction of the rotational tool 214 to the opposite direction as described above.